<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>
        var a = 10; //a = 10
        ++a; // ++a = 11 | a = 11
        var b = ++a + 2; // ++a = 12
        console.log(b); //14


        var c = 10; // c = 10
        c++; // c++ = 11 | c=11
        var d = c++ + 2; // c++ = 11 | c=12
        console.log(d); // 13


        var e = 10;
        var f = e++ + ++e; // 1.e++ = 10 | *e = 11
        console.log(f); // 2. ++e = 12 | e = 12
        // 结果：22
        // 前置自增，先自加，后返回值。
        // 后置自增，先返回原值，再自加。
    </script>
</body>

</html>